Back to the topic.
I am 58 and have been sailboarding for 29 years. Weigh about 72kg and 177cm tall. I would say pretty fit for my age. (Recently played a season of Water Polo for the local 3rd Grade Mens team). Swimming is pretty good too, but I prefer not to get into trouble, than have to swim in even though I have done this a couple of times. Funny thing is I have almost drowned on three separate occasions, two when sailing in waves and one when 7 months old in the bathtub. No bad lasting effects...I think...duh.
Slalom sailing is no problem A.T.M. I can go for a few hours and did 7 days in a row during summer this season. No major physical issues except for extra calluses on the hands. A small price to pay.
Freeride is also OK with no physical soreness afterward usually, but I do a few post sailing stretches given to me by a young female physio for the forearms and tendons.
Wave sailing however is a different story - if there are jumps involved then I have to be careful as I have had more injuries doing this than from any other form of sailing. The more jumps - the more soreness later on even if I land them, if I don't then it does hurt. If I end up doing wave riding with limited jumps, then the physicality of it is a lot less.
My dad had a heart attack at 55 even though he was an extremely fit man. He overdid it doing a 10km fun run during one summer and ended up with only 30% efficiency left in his heart when released from hospital. He did make it to 76 so I am reasonably confident I still have a few good years left in me. 
oh yeah..i've got 11 more years to get to your age Wbailey and another 10 yrs for those blokes at Kyeemagh and with good tender female physio hands I just might get out another 10 
Harden up guys, I have several GPS team members in retirement age, a couple in their 70's. They are vital members who sail every chance they get, even travelling far and wide to do so and never complaining even though they have some health issues as you would.
My good mate Hoppy Bob is in his late 60's, has had a hip removed and has a bone infection, he has 2 legs but really only one doing the work. Well yesterday he sailed and got our team on the board for the month, I'm just so absolutely gob smacked he managed it and so proud he's in my team 
www.gpsteamchallenge.com.au/sailor_session/show?date=2011-05-04&team=20
As a guide - look at the top Masters in bicycle and motorbike racing. The Wollongong and Dobroyd longboarders include guys in their 60s who still go down the Eastern Creek main straight at 220 kmh and also do moto-X. And the top Masters bicycle racers are really quick - even at 65 years or more they seem to beat most of the 18-35 year olds racers and completely blow by the average non-racing kid.
At 48, I'm not seeing any real issues. However, I don't do wavesailing much these days but that's partly because of an injury I got when I was about 23.
The bicycle racers are inspiring (especially when the 65+ world champ blasts past those of us who are mid pack in regional open events) and using them as a guide, I hope to be doing slalom etc into the 70s with appropriate luck and training.
My grandpa is 92 this year and can put his palms on the floor with his legs straight and together. Still has all his marbles and is fit and healthy.
He still gets up every morning at 5 and does his exercises and it obviously has worked.
The most important thing is to keep exercising and keep using your brain.
It really is a case of " if you don't use it you lose it" once you start getting old.
Since someone brought up cycling, I'll quote the commonest piece of advice given on cycling forums.
"HTFU, princess"
They are a sympathetic bunch.
.
Never to old and your never to young
Never and pain are simply a state of mind.
Pain is simply the aftermath of the never again
The rush must be greater than and or equal to the pain to continue to gain
Been doing the running for my sons U14 footy team. Got asked to have a kick with the boys in the over 35's. Cant wait, Go the mighty Newcastle city footy club. From what I can see I won't be anywere near the oldest in the outfit. Hope its not to serious.
Sail smarter not harder
I do hear you .............
Gotta send me local GP a bottle of wine
